Easy derivation of the formula relating the fluctuations of a binary system to the X-ray scattering intensity extrapolated to s = 0

Bhatia & Thornton [Phys. Rev. B. (1970), 2, 3004-3012] have derived the formula that relates the scattering intensity of a binary system to the Fourier transform of the local number density and concentration. For the intensity at the limit of zero scattering angle, the same formula can be derived by a much simpler process.